Intercell interference mitigation

ABSTRACT

Methods and apparatus are described for mitigating intercell interference in wireless communication systems utilizing substantially the same operating frequency band across multiple neighboring coverage areas. The operating frequency band may be shared across multiple neighboring or otherwise adjacent cells, such as in a frequency reuse one configuration. The wireless communication system can synchronize one or more resource allocation regions or zones across the multiple base stations, and can coordinate a permutation type within each resource allocation zone. The base stations can coordinate a pilot configuration in each of a plurality of coordinated resource allocation regions. Subscriber stations can be assigned resources in a coordinated resource allocation region based on interference levels. A subscriber station can determine a channel estimate for each of multiple base stations in the coordinated resource allocation region to mitigate interference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/362,564, filed Nov. 28, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/099,255, filed Dec. 6, 2013, which issued asU.S. Pat. No. 9,509,459 on Nov. 29, 2016, which is a continuation ofU.S. patent application Ser. No. 13/472,040, filed May 15, 2012, whichissued as U.S. Pat. No. 8,626,072 on Jan. 7, 2014, which is acontinuation of U.S. patent application Ser. No. 12/327,732, filed Dec.3, 2008, which issued as U.S. Pat. No. 8,204,442 on Jun. 19, 2012, whichclaims the benefit of U.S. Provisional Application No. 60/992,237, filedDec. 4, 2007, and claims the benefit of U.S. Provisional Application No.61/029,258, filed Feb. 15, 2008, which are incorporated by referenceherein as if fully set forth.

BACKGROUND I. Technical Field

This disclosure relates to the field of wireless communications. Moreparticularly, the disclosure relates to intercell interferencemitigation.

II. Related Art

Wireless communication systems are continually seeking to optimizesystem performance while decreasing interference. The capacity of awireless communication system can be improved by fully utilizing theavailable operating bandwidth. In cellular-type wireless communicationsystems, each cell is typically configured to operate in a predeterminedoperating band. Frequency reuse refers to the ability of a system toassign the same operating band to multiple cells, sectors, or otherdefined coverage areas. A maximum utilization of the available operatingband implements a frequency reuse of one, where every cell operates overthe same operating band. However, transmissions within a given celloften contribute to interference experienced by users in adjacent oroverlapping cells, particularly contributing to the interference inthose overlapping coverage areas occupying the same frequency spectrum.Many cellular-type wireless communication systems implement a frequencyreuse plan that does not fully utilize operating bandwidth in each cellin order to reduce intercell interference. For example, more sparselypopulated spectrum reuse schemes may implement a frequency reuse ofthree or frequency reuse of six. However, the inability to utilize thecomplete operating band limits system capacity and does not eliminatethe possibility of inter-system sources of interference.

BRIEF SUMMARY

Methods and apparatus are described for mitigating intercellinterference in wireless communication systems utilizing substantiallythe same operating frequency band. The operating frequency band may beshared across multiple neighboring or otherwise adjacent cells, such asin a frequency reuse one configuration. The wireless communicationsystem can synchronize one or more resource allocation zones across themultiple base stations, and can coordinate a permutation type withineach resource allocation zone. The permutation type for each resourceallocation zone can be selected from a group of predetermined types. Inthe wireless communication system, channel information can be estimatedfrom all synchronized base stations. In one specific embodiment, eachbase station can transmit or otherwise communicate a pseudo randombinary sequence or identifier and a permutation base for each of theresource allocation zones such that the other synchronized basestations, as well as subscriber stations may be aware of theinformation. Neighboring base stations may share a resource allocationmap, such as a subchannel bit map to facilitate coordination ofsubchannel allocations.

In one aspect, the disclosure includes a method of intercellinterference mitigation. The method includes synchronizing a downlinkresource allocation zone across a plurality of base stations,coordinating a same resource permutation type across the plurality ofbase stations in the downlink resource allocation zone, and allocatingdownlink resources to a subscriber station in the downlink resourceallocation zone.

In another aspect, the disclosure includes a method of intercellinterference mitigation. The method includes coordinating a pilotconfiguration within a coordinated downlink allocation region for aserving base station and each of a plurality of neighboring basestations, determining a neighbor list identifying each of the pluralityof neighboring base stations, and broadcasting, by the serving basestation, pilot data for the pilot configuration and the neighbor list toat least one subscriber station within a coverage area of the servingbase station.

In another aspect, the disclosure includes a method of intercellinterference mitigation. The method includes receiving, at a basestation, a channel quality indicator value from a subscriber station,analyzing the channel quality indicator value, determining a selectedresource allocation region in a communication time frame from aplurality of resource allocation regions that includes at least onecoordinated downlink resource allocation region, and assigning thesubscriber station to the selected resource allocation region.

In another aspect, the disclosure includes a method of intercellinterference mitigation. The method includes classifying a subscriberstation as interference limited, and allocating a downlink resource tothe subscriber station within a coordinated resource zone of a downlinksubframe.

In another aspect, the disclosure includes a method of intercellinterference mitigation. The method includes receiving over the airsignaling messages from each of a plurality of base stations coordinatedacross a coordinated downlink zone, receiving a downlink resourceallocation in the coordinated downlink zone, generating a channelestimate for each of the plurality of base stations based at least inpart on the over the air signaling messages, receiving signals in thedownlink resource allocation, and decoding the signals in the downlinkresource allocation with interference mitigation based on the channelestimate for each of the plurality of base stations.

In another aspect, the disclosure includes a base station configured forinterference mitigation. The base station includes an administrativemanager configured to coordinate at least one downlink resourceallocation region within a communication time frame with a neighboringbase station, a subscriber station capabilities monitor configured tostore capabilities of at least one subscriber station within a coveragearea of the base station, a coordinated downlink zone manager configuredto determine a resource allocation region from a plurality of resourceallocation regions for a subscriber station based at least in part oncapabilities of the subscriber station stored in the subscriber stationcapabilities monitor, and a scheduler configured to schedule informationfor the subscriber station in the resource allocation region.

In another aspect, the disclosure includes a subscriber stationconfigured for interference mitigation. The subscriber station includesa channel estimator configured to determine a channel estimate for eachof a plurality of base stations having coordinated pilot configurationin a coordinated downlink zone, and an interference canceller configuredto mitigate interference within information received within thecoordinated downlink zone based on the channel estimates.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of embodiments of the disclosurewill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings, in which like elements bearlike reference numerals.

FIG. 1 is a simplified functional block diagram of an embodiment of awireless communication system.

FIG. 2 is a simplified functional block diagram of an embodiment of abase station.

FIG. 3 is a simplified functional block diagram of an embodiment of asubscriber station.

FIG. 4 is a simplified flow chart of an embodiment of a method ofcommunicating using a coordinated downlink zone.

FIG. 5 is a simplified flow chart of an embodiment of a method ofprocessing received signals in a coordinated downlink zone.

FIG. 6 is a simplified flow chart of an embodiment of a method ofdetermining a downlink resource allocation zone.

DETAILED DESCRIPTION

Methods and apparatus for intercell interference mitigation aredescribed herein. The methods and apparatus may be applicable to severaldifferent types of wireless communication systems that are cell-based orotherwise service area-based. Without limiting the generality, themethods and apparatus for intercell interference mitigation can beapplied to an Orthogonal Frequency Division Multiple Access (OFDMA)wireless communication system. Further, specific proposed changes to anOFDMA wireless standard, IEEE 802.16 Rev 2/D2, hereby incorporated byreference herein in its entirety, are provided to enable the intercellinterference mitigation methods and apparatus described herein to beimplemented within an existing wireless system.

Although the methods and apparatus described herein are presented in thecontext to an OFDMA wireless communication system, and in particular awireless communication system operating in accordance with an IEEE802.16standard, the intercell interference methods and apparatus are notlimited to application in such a system, and may be applicable to otherwireless communication systems. For example, the methods and apparatusfor interference mitigation described herein may be applicable towireless systems such as, but not limited to, Long Term Evolution (LTE),Wireless Local Area Networks (WLAN), Digital Video Broadcasting (DVB),and the like. The examples described herein are described in the contextof multi-carrier systems, such as Orthogonal Frequency DivisionMultiplexed (OFDM) systems, but the disclosure is not limited toapplication in OFDM systems. Furthermore, the methods and apparatusdescribed herein utilize at least two dimensions for resourceallocations (time and frequency). The embodiments described herein,however, are not limited to any particular number of dimensions, but areapplicable to any number of resource allocation dimensions.

Enabling frequency reuse-1 is very important for a wide adoption of802.16e OFDMA PHY (orthogonal frequency division multiple accessphysical layer) based systems, as spectrum efficiency is one importantperformance measure for competing wireless technologies. Intercellinterference has been identified as a major problem for effectivelyachieving frequency reuse-1. Therefore, managing interference becomesone of the most important elements to improve the system performance andhence competitive advantage of 802.16e based systems. However, theinterference mitigation methods and apparatus described herein are notlimited to application in a wireless communication system implementingfrequency reuse-1, but is also applicable to other frequency reuseimplementations, such as frequency reuse-3, frequency reuse-6, or someother frequency reuse plan.

Interference management can be achieved by a combination of interferenceavoidance and interference cancellation techniques. Most interferencemitigation techniques are based on the knowledge of the interferenceproperties. However, the current 802.16e specification does not provideadequate support for making the interference property knowledgeavailable for effective implementations of interference mitigationschemes. The techniques, methods, and apparatus described herein can beimplemented in an 802.16 system with a few minor and backward compatiblechanges into the 802.16 Rev2/D3 standard that will provide adequatesupport to aid interference mitigation schemes for the OFDMA PHY basedsystems.

Base station signaling and coordination of resource allocation can beused to reduce downlink interference and to facilitate downlinkinterference mitigation techniques implemented within a subscriberstation. The base station may assign uplink resources to subscriberstations based on an interference classification in order to reduce thepotential for uplink interference and to improve uplink throughput,especially in interference limited scenarios.

The subscriber station may implement interference mitigation techniqueson the downlink signals based on knowledge of the downlink signalingcharacteristics. For example, the subscriber station can be configuredto estimate the channel characteristics for a predetermined number ofcommunication channels and utilize the channel characteristics that areestimated for interference channels to mitigate the effects ofinterference on the received signals.

FIG. 1 is a simplified functional block diagram of an embodiment of awireless communication system 100. The wireless communication system 100includes a plurality of base stations, 110-1 and 110-2 coupled to anetwork 114, such as a wide area network. Each base station, e.g. 110-1,services devices within its respective coverage area, e.g., 112-1,sometimes referred to as a cell.

A first base station 110-1 serves a first coverage area 112-1 and asecond base station 110-2 serves a corresponding second coverage area112-2. The base stations 110-1 and 110-2 are depicted as adjacent orotherwise neighboring base stations for the purposes of discussion.Similarly, the coverage areas 112-1 and 112-2 are shown as overlappingin order to facilitate the description of intercell interference andintercell interference mitigation.

The depiction of the first and second base stations 110-1 and 110-2 andtheir respective coverage areas 112-1 and 112-2 as adjacent or otherwiseoverlapping is provided to facilitate discussion of interference. Ofcourse, the second base station 110-2 need not be adjacent the firstbase station 110-1 in order for their respective coverage areas 112-1and 112-2 to overlap. Additionally, the coverage areas 112-1 and 112-2need not overlap in order for signals from one of the base stations,e.g. 110-2, to operate as a source of interference for signalstransmitted or received by another base station, e.g. 110-1.

The first and second base stations 110-1 and 110-2 can be configuredusing a reuse-1 frequency reuse plan, or some other frequency reuseplan, that results in the two base stations 110-1 and 110-2 utilizingthe same or at least a portion of the same operating band. Althoughemissions from a first wireless device always present some type ofinterference to neighboring devices, the interference is particularlytroublesome in wireless communication systems that rely at leastpartially on frequency selectivity for channelization of communications.The wireless devices may not have sufficient capabilities to distinguishmultiple communications within the same frequency channel, and thusinterference within the operating frequency band may significantlydiminish signal quality.

As an example, the base stations 110-1 and 110-2 serve those deviceswithin the respective coverage areas 112-1 and 112-2. As shown in FIG.1, first and second client stations or subscriber stations 120 a and 120b are within the first coverage area 112-1 and can be supported by thefirst base station 110-1.

In the example where the wireless communication system is configuredwith a frequency reuse-1, both the first base station 110-1 and thesecond base station 110-2 operate over substantially the same operatingband. Thus, a wireless device, such as the first subscriber station 120a operating in an overlapping coverage area, experiences interference asa result of transmissions by the second base station 110-2 on the samefrequency.

The wireless communication system 100 can mitigate the effects ofintercell interference by establishing one or more coordinated downlink(DL) zones. Each coordinated DL zone is a portion of the downlinkresources (in the direction from base station to subscriber station)that is coordinated across multiple base stations. The neighboring basestations may have predetermined coordinated DL zones and may be able todynamically define additional coordinated DL zones, as needed.

The term “zone,” as used herein, refers to a logical portion of thesignal and does not refer to a physical area or geographic area. Thezone may represent a portion of available resources. For example, in aWiMax wireless communication system that operates in accordance withIEEE 802.16 standard, the available resources include time, frequency,space, power, or some combination thereof. A coordinated DL zone maycoordinate a logical portion of any one or more of the availableresources.

Furthermore, although the term “zone” is used in a manner consistentwith the usage in a WiMax wireless communication system, the term “zone”is not limited to use in a WiMax configuration, but instead, refers toany logical partition of resources. Thus, the term “zone” is usedconsistently with the terms “region,” “partition,” “allocation space,”“subframe portion,” “packet,” “division,” and the like, used to conveythe partitioning and allocation of resources according to distinctincrements.

In one embodiment, a base station 110-1 can determine a level ofinterference experienced by a subscriber station, e.g. 120 a, andallocate downlink resources in the coordinated downlink zone to thesubscriber station for communicating the downlink information based onthe level of interference. In one embodiment, the base station 110-1 candetermine the level of interference based on a feedback interferencemetric value communicated from the subscriber station 120 a to the basestation 110-1 or otherwise determined from one or more metricscommunicated from the subscriber station 120 a to the base station 110.

For example, the base station 110-1 can be configured to receive achannel quality indicator (CQI) value from each of the subscriberstations 120 a and 120 b, for which it is the serving base station. Thebase station 110-1 can perform interference analysis based in part onthe received CQI values. The base station can 110-1 perform interferenceanalysis by comparing particular CQI values against thresholdsassociated with the CQI type. The different CQI types can include, forexample, Signal to Interference Ratio (SIR), Interference to Noise Ratio(INR), Carrier to Interference Ratio (CIR), and the like.

The base station 110-1 can, for example, compare a SIR CQI value againsta first threshold and determine that an interference condition exists ifthe SIR value does not exceed the threshold value. Similarly, the basestation 110-1 can compare an INR CQI value against a second thresholdand determine that an interference condition exists if the INR valueexceeds the threshold value. In another embodiment, the base station110-1 can perform interference analysis based on a plurality of CQIvalues corresponding to a plurality of CQI types. For example, the basestation 110-1 may receive a SIR CQI value and a INR CQI value from asubscriber station and can determine that an interference conditionexists if the SIR value does not exceed a first threshold value and theINR value exceeds a second threshold value.

The base station 110-1 may operate on each CQI value individually, ormay further process the received CQI values. The base station 110-1 can,for example, filter the received CQI values by determining an average,moving average, weighted average, windowed filter, and the like, onreceived CQI values.

In another embodiment, one or more of the coordinated downlink zonesrepresents a Cell Edge Zone (CEZ). The base stations 110-1 and 110-2 canallocate downlink resources within the CEZ to support subscriberstations, e.g. 120 a, that are determined to be at the edges of a cell.A base station, e.g. 110-1, can determine if a subscriber station, e.g.120 a, is at an edge of a cell or coverage area 112-1, and can allocatedownlink resources within the CEZ based on the determination. Asubscriber station, e.g. 120 a, at a cell edge can be presumed to beexposed to a greater level of interference due to a closer proximity toadjacent base stations, e.g. 110-2, that are potential sources ofinterference.

The coordinated DL zone includes at least a portion of the downlinkfrequency band that is shared between the two base stations 110-1 and110-2. In an Orthogonal Frequency Division Multiple Access (OFDMA)system, the downlink resources may be allocated in two dimensions, timeand frequency. Thus, a coordinated DL zone in an OFDMA system may bedefined in terms of time and frequency. As an example, OFDMA frames maybe defined to be a predetermined number of OFDM symbols synchronizedwithin a predetermined period of time. The coordinated DL zone can bedefined in terms of the symbol indices and subcarrier indices within thesymbols of the OFDMA frame.

The coordinated DL zone may be positioned virtually anywhere in adownlink frame. In some embodiments, the coordinated DL zone can bepositioned adjacent in time to an overhead signaling portion of thedownlink frame that includes, for example, a broadcast preamble. Asubscriber station can determine a channel estimate based in part on thepreamble and a temporal proximity of the coordinated DL zone to thepreamble can improve the correlation between the channel estimate andthe actual channel conditions that exist during the coordinated DL zone.

In the coordinated DL frame, the first and second base stations 110-1and 110-2 coordinate or otherwise synchronize various parameters. Theseparameters include, for example, the zone boundaries, a permutation typeimplemented within the coordinated DL zone, and a modulation scheme usedto convey information in the coordinated DL zone. The coordination ofthe various aspects of the coordinated DL zone may be coordinated inreal time through message exchange or feedback. Alternatively, thecoordination of the coordinated DL zone across the plurality of basestations need not be performed in real time, but instead, may beestablished as a configuration parameter.

Additionally, the base stations 110-1 and 110-2 may inhibit or otherwisedeactivate certain functionality during the coordinated DL zone. Forexample, the base stations 110-1 and 110-2 may inhibit or otherwiserefrain from MIMO transmission, and may inhibit or otherwise refrainfrom assigning any dedicated pilot signals in the coordinated DL zone.

A permutation type may refer to a particular algorithm or process forassigning and permuting the subcarriers that are allocated to aparticular subscriber station. The permutation type can limit the numberof subcarriers that may be assigned and can control a type of codingthat is applied to the signals. The subcarriers of a particularpermutation type may be scrambled or otherwise permuted according to apermutation scheme that can rely on a permutation base and apseudorandom binary sequence PRBS. The combination of the permutationtype, permutation base, and PRBS can be used to predict or otherwisedetermine the subcarriers for a particular frame or symbol. Thepermutation type can be viewed, for example, as a mapping from a logicalchannel to a physical channel embodied in the OFDMA symbols. Althoughthe permutation type is described in the context of an OFDMA system, thepermutation type may represent any type of logical to physical mapping,and the mapping is not limited to application within an OFDMA system.Each base station can communicate its downlink PRBS and a PRBSidentification in one or more fields of an overhead message or messages.

Examples of permutation types can include, but are not limited to, PUSC(partial usage of subchannels), STC PUSC (space time coding partialusage of subchannels), AMC (adaptive modulation and coding), STC AMC(space time coding adaptive modulation and coding), and FUSC (full usageof subchannels).

Additionally, the first and second base stations 110-1 and 110-2 maycoordinate the resource allocations within the coordinated DL zone. Forexample, each of the first and second base stations 110-1 and 110-2 canmanage or otherwise track resource allocations in the coordinated DLzone by updating a used subchannel bitmap. The first and second basestations 110-1 and 110-2 reduce or otherwise eliminate concurrent usageof the same subchannels or OFDMA subcarriers through the use andmaintenance of the used subchannel bitmap.

The first and second base stations 110-1 and 110-2 can operate using oneor more coordinated DL zones. Each base station, e.g. 110-1 and 110-2,supporting coordinated DL zones can determine whether a subscribestation within its respective coverage area supports interferencemitigation by signal processing of signals within a coordinated DL zone.The first and second base stations 110-1 and 110-2 can assign asubscriber station, e.g. 120 a to the coordinated DL zone in order tomitigate the effects of intercell interference. Subscriber stations,such as second subscriber station 120 b, not subject to the debilitatingeffects of intercell interference or not configured to provideinterference mitigation of signals within a coordinated DL zone need notbe allocated resources within a coordinated DL zone.

Although virtually any subscriber station, e.g. 120 a, may benefit frombeing assigned to a coordinated DL zone when operating in the presenceof intercell interference, a subscriber station, e.g. 120 a, havingintercell interference mitigation capabilities may further benefit byhaving knowledge of operation in a coordinated DL zone.

The serving base station, e.g. 110-1 can query the first subscriberstation 120 a for its capabilities. The first base station 110-1 can,for example, send a subscriber station capabilities query to the firstsubscriber station 120 a and based on the response, can configure the DLresource allocation and coordinated DL zone assignment. For example, thefirst base station 110-1 can query the first subscribe station 120 a forresource permutation type capabilities and determine the supportedresource permutation type in a subscriber station response to the query.The first base station 110-1 can then allocate to the subscriber stationa resource permutation in the coordinated DL zone based on the supportedresource permutation type.

In one embodiment of a wireless communication system based in part onIEEE 802.16, a DL zone can be configured as a coordinated DL zone, wherethe serving base station and one or more neighbor base stations have thesame zone boundary, the same zone permutation type e.g., PUSC, STC PUSC,AMC, STC AMC, and FUSC, and the same values for operating parameters,such as “Use All SC” and “Dedicated Pilots.” Some systems may permitpilot boosting, where the pilot channels are broadcast at a higher powerlevel. Within a coordinated DL zone, the resource allocation can beconfigured to have the parameter “boosting” set to 0b000, i.e., notboosted. The inhibiting of boosted pilot channels reduces the likelihoodthat the subscriber station will generate an inaccurate channel estimatebased on boosted pilot signals.

A frame of multiple symbols can have zero, one, or multiple coordinatedDL zones. A first PUSC zone can also be configured as a coordinated DLzone. When the first PUSC zone is a coordinated DL zone, the servingbase station coordinates with one or more neighbor base stations to havethe same zone boundary and use the same “used-subchannel bitmap.”

Each base station configured to support a coordinated DL zone with oneor more base stations can be configured to coordinate one or moreoperating parameters within the coordinated DL zone. The coordinatedoperating parameters can include, for example, the state of a “use allsubchannels” parameter as well as the state of a “dedicated pilots”parameter. For a zone defined by the downlink zone information elementas a coordinated DL zone, the “use all subchannels” field can be set tothe same value for all base stations that are coordinated in thecorresponding DL coordinated zones. Similarly, for a zone defined by thedownlink zone information element as a coordinated DL zone, the“Dedicated Pilots” field can be set to the same value for all basestations that are coordinated in the corresponding DL coordinated zones.

A serving base station, e.g. 110-1, can allocate available resources inthe coordinated DL zone to a subscriber station, e.g. 120 a. The servingbase station, e.g. 110-1, can verify or otherwise determine that DLresources are available in the coordinated DL zone by referring to aused-subchannel bitmap. The serving base station, e.g. 110-1, canallocate the DL resources in the coordinated DL zone by sending aDownlink Channel Descriptor (DCD) management message of a type andlength associated with coordinated DL zone allocations. The fields orvalues in the DCD management message can be set based on the particularDL resource allocation. The DCD management message can includeinformation for notifying the presence of one or more coordinated DLzones. Different wireless systems may implement similar managementmessages for communicating this type of information, and theinterference mitigation techniques described herein are not limited toresource allocations following any specific type of management message.

Broadcasting the presence and associated timing of one or morecoordinated DL zones permits subscriber stations to implementinterference mitigation techniques on signals received in thecoordinated DL zones. For example, the first base station 110-1 can senda DCD management message having the type, length, and values as setdefined in Table 1.

TABLE 1 Name Type Length Value PHY Scope DL 62 1 Bit#0: the coordinatedfirst DL PUSC OFDMA Coordinated zone indication, if set to 1, Zoneindicates the first DL PUSC zone is indication a coordinated zone. Ifset to 0, indicates the first DL PUSC zone is not a coordinated zone.Bit#1: the coordinated second DL zone indication, if set to 1, indicatesthe second DL zone is a coordinated zone. If set to 0, indicates thethird DL zone does not exist or is not a coordinated zone. Bit#2: thecoordinated third DL zone indication, if set to 1, indicates the thirdDL zone is a coordinated zone. If set to 0, indicates the third DL zonedoes not exist or is not a coordinated zone. Bit#3: the coordinatedfourth DL zone indication, if set to 1, indicates the fourth DL zone isa coordinated zone. If set to 0, indicates the fourth DL zone does notexist or is not a coordinated zone. Bit#4: the coordinated fifth DL zoneindication, if set to 1, indicates the fifth DL zone is a coordinatedzone. If set to 0, indicates the fifth DL zone does not exist or is nota coordinated zone. Bit#5 DO coordinated STC PUSC zone matrix indicator,if set to 1, indicates Matrix A only in the first DL STC PUSC zone. Ifset to 0, indicates not Matrix A only. Bit#6 DL coordinated STC AMC zonematrix indicator, if set to 1, indicates Matrix A only in the first DLSTC AMC zone. If set to 0, indicates not Matrix A only. Bit#7: reserved

Each base station configured to support a coordinated DL zone canbroadcast additional information that facilitates interferencemitigation at subscriber stations. The broadcast information can includea neighbor list, which can identify those neighboring base stationssupporting the coordinated DL zones, and can include an unambiguousidentifier for the neighboring base stations. The base stations in awireless system may be identified with an identification number that maynot be unique across the entire system. However, the base stationswithin a neighboring geographic region may be uniquely or otherwiseunambiguously identified within the particular geographic region usingthe numbers. A base station may broadcast the unambiguous identifiersfor the neighboring base stations in a neighbor list. The subscriberstation can utilize the neighbor list for purposes of handoff and tofacilitate acquiring signals from the neighboring base stations.

The broadcast information can include, for example, the value or mannerof determining a downlink permutation base (DL_PermBase) for eachcoordinated DL zone and the value or manner of determining a pseudorandom binary sequence identification (PRBS-ID), which is used to by asubscriber station to determine subcarrier numbering. In one embodiment,the DL_PermBase field shall be set to the five Least Significant Bits(LSBs) of an IDcell value indicated by a frame preamble when the zonedefined by a downlink zone information element, e.g. STC_DL_Zone_IE( )is a coordinated DL zone. The PRBS_ID field can be set to mod(segmentnumber +1, 3) as indicated by the frame preamble when the zone definedby the downlink zone information element is a coordinated DL zone.

The first base station 110-1 is not limited to assigning subscriberstations to coordinated DL zones based on operation within anoverlapping portion of the coverage area or operation at a cell edgezone. Instead, the first base station 110-1, and in general any basestation supporting the coordinated DL zones, can schedule resources in acoordinated DL zone and can assign a subscriber station to a coordinatedDL zone based on a number of parameters. The parameters can include, forexample, the base station signal strengths observed at the subscriberstation, or some other parameter that may be indicative of intercellinterference.

The base station may also implement methods of avoiding uplinkinterference between subscriber stations. Interference avoidance in theuplink path is different from interference avoidance in the downlinkpath because a single base station may be configured to concurrentlyreceive uplink signals from a plurality of subscriber stations that mayeach be positioned virtually anywhere within the coverage area supportedby the base station.

In the uplink, the antenna gain differences between boresight and sectoredges can be greater than about 10 dB. Thus, interference can be largelydetermined based on the locations of the desired and interferingsubscriber stations. A subscriber station that is farther out but in theboresight of a neighboring base station will be received at a strongerpower than a subscriber station that is positioned at a cell edge, butpositioned much closer than the subscriber station in the antennaboresight.

Moreover, in some wireless communication systems, such as thoseoperating in accordance with IEEE 802.16, uplink resource allocationsare tiled in the frequency domain first. This ordering of resourceallocations makes separation of subscriber station allocations in thetime domain more difficult. Implementing zone switching in the uplinkreduces the power density of the subscriber station transmissions due tothe increase in the number of subchannels that are occupied infrequency.

To improve coverage and throughput in the uplink, the base station canidentify and separate subscriber stations based on their locations inthe coverage area. For example, those subscriber stations at cell edgescan be separated from other subscriber stations for the purposes ofuplink resource allocations. Furthermore, the types of MIMO andmodulation modes assigned to the cell edge subscriber stations may belimited or otherwise controlled.

The base station can divide subscriber stations into a predeterminednumber of groups. Each subscriber station can be characterized as, forexample, one of a cell-edge, sector-edge, or cell-center. The basestation can assign orthogonal resources to the different population sets(e.g. cell-edge, sector-edge, or cell-center) so that interference tousers within a particular population set is substantially limited tothose subscriber stations belonging to the same population set. Theorthogonal resources can include, but are not limited to, power, time,space, frequency, or some combination thereof. In one embodiment, thebase station can separate the users of the population sets by frequency,and can assign distinct subchannel sets to each of the population sets.

The base station assignment of orthogonal resources to the distinctpopulation sets operates to reduce the uplink interference experiencedby different subscriber stations in the same coverage zone, but may notminimize the interference in the uplink of neighboring base stations.The lack of coordination of uplink resource allocation with neighboringbase stations may only present interference issues in those wirelesscommunication systems implementing reuse-1. In more sparsely populatedreuse schemes, such as reuse-3, the uplink signals are more likely to benoise limited rather than interference limited.

One embodiment to further reduce uplink interference experienced byneighboring base stations is for a base station to limit or otherwisecontrol a modulation coding scheme (MCS) assigned to those subscriberstations in the cell-edge and sector-edge population sets. For example,the base station may limit the MCS of uplink resource allocations to MCStypes that minimize a rise-over-thermal noise contribution (referred toas ‘IoT’) experienced at neighboring base stations. Optimally, theserving base station would know or estimate the path loss differencebetween the serving base station and the strongest interferer in orderto assign a MCS to minimize IoT experienced at the neighboring basestations. In the absence of path loss knowledge, the serving basestation can estimate the strongest interferer based on a metric, such asa preamble carrier-to-interference and noise (CINR) value or some otherChannel Quality Indicator that may be reported to the base station. Tofurther reduce the possibility of uplink interference, each base stationshould coordinate its ranging opportunities with the neighboring basestations.

FIG. 2 is a simplified functional block diagram of an embodiment of abase station 110. The base station can be, for example, the first orsecond base station 110-1 or 110-2, respectively, shown in the wirelesscommunication system of FIG. 1. The base station 110 includes thecapabilities to configure and support one or more coordinated DL zoneswith neighboring base stations. The base station 110 functionality issimplified to include those portions that operate as part of intercellinterference mitigation. Other portions of the base station 110 areomitted for the purposes of brevity and clarity.

The base station 110 includes a transmit or downlink processing path anda receive or uplink processing path. The downlink processing pathincludes a data source 210 that can be configured to generate orotherwise interface with a data generator. The data source 210 iscoupled to a scheduler 290 that includes a downlink baseband processor220 that also functions as a signal mapper. An administrative manager212 can coordinate the coordinated DL zones with the neighboring basestations. For example, the administrative manager 212 can coordinate thepilot configuration across the multiple coordinated base stations. Theadministrative manager 212 can store or otherwise access an externalsubchannel bit map (not shown) used when allocating resources in acoordinated DL zone. The administrative manager 212 can, for example,store the identities of the neighboring base stations in a neighborlist. The administrative manager 212 can couple the neighbor list to thescheduler 290 for subsequent broadcast.

The scheduler can also be configured to schedule uplink resources forthe subscriber stations in the base station 110 coverage area. Theoutput of the scheduler 290 is coupled to a transmit path of an RFtransceiver 230.

The downlink RF signal is coupled to an antenna controller 240 thatoperates to control a plurality of antennas 242-1 and 242-n in order toimplement some type of diversity, such as space time diversity,beamforming, beamsteering, and the like or some other type of antennasignal distribution and control.

In the uplink direction, the uplink signals from the subscriber stations(not shown) are coupled to the antennas 242-1 and 242-n via the antennacontroller 240 to a receive portion of the RF transceiver 230.

A coordinated DL zone manager 250 is coupled to the RF transceiver 230,a subscriber station capabilities monitor 260, and a permutation manager270. The permutation manager 270 includes a permutation base storage 272and a pseudorandom binary sequence ID storage 274. The permutationmanager 270 is coupled to the downlink baseband processor 220, forexample, to control the signal mapping in the coordinated DL zones.

As described above, the wireless system 100 can be configured toestablish one or more coordinated DL zones and synchronize the zonesacross multiple base stations. Thus, the base station 110 synchronizedwith a coordinated DL zone can allocate resources to a subscribe stationin the coordinated DL zone.

The base station 110 can register a subscriber station, for example,when the subscriber station enters the coverage area supported by thebase station 110, and send a query to the subscriber station for itscapabilities, including the ability to support the coordinated DL zones,and the permutation types supported by or otherwise enabled in thesubscriber station. Specifically, within the base station 110 thecoordinated DL zone manager 250 can receive from the subscriber stationcapabilities monitor 260 a query directed to the subscriber station todetermine the subscriber station capabilities.

The base station 110 can receive the response from the subscriberstation, and the coordinated DL zone manager 250 can store, update, orotherwise access the subscriber station capabilities, which may bestored, for example, within a look up table in the subscriber stationcapabilities monitor 260.

The base station 110 can receive one or more parameters or requests fromthe subscriber station that indicate a desire or advantage for assigningthe subscriber station to a coordinated DL zone. The subscriber stationmay make an express request or the desire to transition a subscriberstation to a selected coordinated DL zone may be implicit from one ormore messages or information received from the subscriber station. Thebase station 110 can be configured to receive CQI values from eachsubscriber station for which it is the serving base station. Thecoordinated DL zone manager 250 can receive and perform interferenceanalysis on the received CQI values for each subscriber station. Thecoordinated DL zone manager 250 can establish a coordinated DL zone, inpart, by allocating DL resources to the coordinated DL zone sufficientto support a particular permutation scheme controlled by the permutationmanager 270.

The coordinated DL zone manager 250 can, for example, configure aparticular permutation type supported by the subscriber station, anddetermine the permutation parameters, including permutation base andPRBS associated with the permutation type. The permutation manager 270can retrieve a permutation base from a permutation base storage 272 andcan determine a PRBS from a PRBS ID storage 274 that can include, forexample, the PRBS values and the corresponding PRBS indices identifyingthe PRBS. The coordinated DL zone manager 250 can generate one or moremessages to the subscribe station and neighboring base stations of thecoordinated DL zone, including the permutation type, permutation base,and PRBS ID.

The coordinated DL zone manager 250 can initialize a used subchannelbitmap and can update the bitmap as subchannels are allocated to thesubscriber stations assigned to the coordinated DL zone. The subchannelscan be subchannels assigned by the base station 110 or some other basestation (not shown) that is participating in the coordinated DL zone.

The coordinated DL zone manager 250 may also control the operation ofone or more other parameters or configurations during the coordinated DLzone. For example, the coordinated DL zone manager 250 may be configuredto control the baseband processor 220 to inhibit or otherwise refrainfrom assigning any dedicated pilots during the coordinated DL zone.Similarly, the coordinated DL zone manager 250 may control the RFtransceiver 230 or antenna controller 240 to inhibit or otherwiserefrain from implementing MIMO configurations during the coordinated DLzone.

FIG. 3 is a simplified functional block diagram of an embodiment of asubscriber station 120. The subscriber station 120 can be, for example,a subscriber station, e.g., 120 a or 120 b, in the wirelesscommunication system of FIG. 1, and can support various permutationtypes and coordinated DL zones in which the various permutation typesmay be implemented.

The subscriber station 120 includes an antenna 306 through which theuplink and downlink signals are communicated. The antenna 306 couplesthe downlink signals to a transmit/receive (T/R) switch 310. The T/Rswitch 310 operates to couple the downlink signals to the receiverportion of the subscriber station 120 during a downlink subframe andoperates to couple uplink signals from the transmitter portion of thesubscriber station 120 during an uplink subframe.

During the downlink portion or subframe, the T/R switch 310 couples thedownlink signals to a receive RF front end 320. The receive RF front end320 can be configured, for example, to amplify, frequency convert adesired signal to a baseband signal, and filter the signal. The basebandsignal is coupled to a receive input of a baseband processor 340.

The receive input of the baseband processor 340 couples the receivedbaseband signal to an Analog to Digital Converter (ADC) 352 thatconverts the analog signal to a digital representation. The output ofthe ADC 352 can be coupled to a transformation module, such as FastFourier Transform (FFT) engine 354 that operates to convert the receivedtime domain samples of an OFDM symbol to a corresponding frequencydomain representation. The sample period and integration time of the FFTengine 354 can be configured, for example, based upon the downlinkfrequency bandwidth, symbol rate, subcarrier spacing, as well as thenumber of subcarriers distributed across the downlink band, or someother parameter or combination of parameters.

The output of the FFT engine 354 can be coupled to a channelizer 356that can be configured to extract the subcarriers from those symbolsthat are allocated to the particular subscriber station 120. Thechannelizer 356 can be configured, for example, with the permutationtype, permutation base, and PRBS associated with the coordinated DL zonein which the subscriber station 120 operates. The output of thechannelizer 356 can be coupled to a destination module 358. Thedestination module 358 represents an internal destination or output portto which received data may be routed.

The channelizer 356 can be configured to determine or otherwise generateone or more channel quality indicator values based on the extractedsubcarriers and channel estimates provided by the channel estimator 370or interference canceller 380.

The subscriber station 120 also includes a channel estimator 370 that iscoupled to the FFT engine 354 and that can, for example, generate orotherwise determine a channel estimate based on pilot subchannelsincluded in the DL symbols. The channel estimator 370 can be coupledwith a permutation manager (not shown) that receives information fromneighboring base stations, such as permutation base, PRBS ID, and soforth. In implementation, the permutation manager can be integrated aspart of the channel estimator 370. The channel estimator can couple thechannel estimates to an interference canceller 380 that operates tocancel, compensate for, or otherwise mitigate the effects of intercellinterference.

The interference canceller 380 can, for example, provide the channelestimates to the channelizer 356 to permit the channelizer to cancel thenoise and interference contribution from the non-serving base stations.Alternatively, the interference canceller 380 can modify or otherwisecontribute to a decoder operation within the channelizer 356.

The uplink path is complementary to the downlink signal path. A sourcemodule 362 of the base band processor 340, which may represent aninternal data source or an input port, generates or otherwise couplesuplink data to the baseband processor 340. A subscriber stationcapabilities module 342 can be used to store the subscriber stationcapabilities, and can be configured to generate a message in response toa query for the subscriber station capabilities. The source 362 couplesthe uplink data to an uplink channelizer 364 that operates to couple theuplink data to appropriate uplink resources that are allocated tosupport the uplink transmission.

The output of the uplink channelizer 364 is coupled to an IFFT engine366 that operates to transform the received frequency domain subcarriersto a corresponding time domain OFDM symbol. The uplink IFFT engine 366may support the same bandwidth and number of subcarriers as supported bythe downlink FFT engine 356.

The output of the uplink IFFT engine 366 is coupled to a Digital toAnalog Converter (DAC) 368 that converts the digital signal to an analogrepresentation. The analog baseband signal is coupled to a transmitfront end 322, where the signal is frequency translated to the desiredfrequency in the uplink band. The output of the transmit front end 322is coupled to the T/R switch 310 that operates to couple the uplinksignal to the antenna 306 during the uplink subframe.

An LO 330 is coupled to a switch 332 or demultiplexer that selectivelycouples the LO 330 to one of the receive front end 322 or transmit frontend 322 so as to be synchronized to the state of the T/R switch 310.

FIG. 4 is a simplified flowchart of a method 400 of communicating usinga coordinated downlink zone. The method 400 can be implemented, forexample, by a base station of the wireless communication systemillustrated in FIG. 1. The method 400 permits the establishment,coordination and signaling associated with a coordinated DL zone, suchas a coordinated DL zone in a TDM OFDMA wireless communication system.

The method 400 begins at block 410 where a base station coordinates theexistence and location of coordinated DL zone with at least one otherbase station. Typically, a base station tracks the identities of theneighboring base stations and may establish and maintain a coordinatedDL zone with one or more neighboring base station.

The various steps of the method 400 are provided in a logical order tofacilitate description of the process of communicating using acoordinated downlink zone. The actual order of the various steps neednot follow the logical order, unless the need for such order is apparentfrom the description.

The base station may coordinate the DL zone by communicating orotherwise coordinating the zone boundaries and zone permutation typewith the neighboring base station. In one embodiment, the zoneboundaries may be, for example, explicitly established in a controlmessage communicated between the coordinating base stations or may bemaintained within each base station as a configuration parameter. Whereconfiguration parameters permit multiple distinct zones, thecoordinating base stations may exchange information regarding which ofthe zones is to be established as a coordinated DL zone.

Alternatively, the zones that are configured as coordinated DL zones maybe established network wide, and configured through one or moreconfiguration parameters. In such an embodiment, the existence andlocation of coordinated DL zones need not be expressly communicated toeach of the base stations.

The base station proceeds to block 420 and coordinates the subcarrierusage including the permutation type that is used in each coordinated DLzone. The base station can coordinate all subcarriers are used and thespecific permutation type. The permutation type can set forth, forexample, the number of subcarriers utilized and type of space timecoding utilized for the downlink signals. The permutation type can beselected from, for example, PUSC (partial usage of subchannels), STCPUSC (space time coding partial usage of subchannels), AMC (adaptivemodulation and coding), STC AMC (space time coding adaptive modulationand coding), and FUSC (full usage of subchannels). The permutation typecan be coordinated between the base stations through express messageexchange or can be established implicitly, for example, viaconfiguration parameters.

The base station proceeds to block 430 and coordinates the pilot signalsused in the coordinated DL zone. In one embodiment, the base station canselect between dedicated pilots and non-dedicated pilots (alternativelyreferred to as broadcast pilots). Each of the base stations coordinatedacross a coordinated DL zone uses the same pilot signaling type. Thatis, if a first base station in the coordinated DL zone utilizesdedicated pilots, then a subscribe station can determine that all otherbase stations configured to support the same coordinated DL zone willalso utilize dedicated pilots. The same is true for the conversecondition. Where a first base station does not utilize dedicated pilotsin the coordinated DL zone, but instead, utilizes broadcast pilots, theother base stations supporting the same coordinated DL zone do not usededicated pilots and also utilize broadcast pilots.

The base station proceeds to block 440 and broadcasts base stationidentifying information, for example, in a neighbor list. The basestation identifying information can include information used by asubscriber station to identify a particular base station as a source ofa signal. Additionally, a subscriber station may utilize the basestation identifying information to estimate a channel from the basestation. The base station identifying information can include, forexample, a downlink permutation base value (DL_PermBase) and a pseudorandom binary stream identification value (PRBS_ID). This informationcan be sent, for example, in a Space Time Coding DL Zone InformationElement (STC_DL_Zone_IE).

The base station proceeds to block 450 and broadcasts base stationconfiguration information that facilitates interference mitigation byinterference mitigation capable subscriber stations. Such informationcan include, for example, the permutation type, the state of a “use allsubchannels” parameter, and the use or absence of dedicated pilots. Thisinformation can also be sent, for example, in a Space Time Coding DLZone Information Element.

The base station proceeds to block 460 and broadcasts informationregarding the existence and location of coordinated DL zones. The basestation can, for example, broadcast a downlink channel descriptor (DCD)management message that is configured with a type and length associatedwith conveying information relating to coordinated DL zone allocations.The fields or values in the DCD management message can be set based onthe particular DL resource allocation and the particular positions orboundaries of the coordinated DL zones. The DCD management message caninclude information for notifying the presence of one or morecoordinated DL zones.

The base station proceeds to block 470 and negotiates the capabilitiesof those subscriber stations for which it is the serving base station.The base station can, for example, determine the subscriber stationcapabilities through a capabilities request and response exchange withthe subscriber station. The base station may receive a DL coordinatedzone capability message from the subscriber station. The DL coordinatedzone capability message from the subscriber station may, for example, beidentified by a predetermined type identifier and may have a length ofone byte. The state of each of the bits in the message may indicate theability to support a particular permutation type in a coordinated DLzone. An example of the type, length, and value parameters for such asubscriber capabilities message is provided in Table 2.

TABLE 2 PHY Name Type Length Value Scope DL 185 1 Bit#0: Support DLcoordinated SBC- Co- zone fo rnon-STC PUSC REQ ordinated Bit#1: SupportDL coordinated Zone zone for STC PUSC if all Capability bursts usesMatrix A Bit#2: Support DL coordinated zone for STC PUSC Bit#3: SupportDL coordinated zone for AMC Bit#4: Support DL coordinated zone for STCAMC if all bursts uses Matrix A Bit#5: Support DL coordinated zone forSTC AMC Bit#6: Support DL coordinated F zone for USC Bit#7: reserved

The base station proceeds to block 480. If the base station is a servingbase station for a particular subscriber station within its coveragearea, the base station receives the downlink data for the subscriberstation. The base station receives the downlink information and formatsit for transmission to the subscriber station. The base station canencode and modulate the downlink information. The base station canformat the data, for example, based on a signal quality and bandwidthrequirements of the subscriber station. The base station may take intoaccount geographic positioning of the subscriber station and levels ofinterference experienced by the subscriber station in formatting thedownlink information for transmission to the subscriber station.

The base station proceeds to block 490 and schedules or otherwiseallocates downlink resources to the subscriber station based on thesubscriber station capabilities. The base station may selectivelyallocate resources within a coordinated DL zone if the subscriberstation supports such a zone and if the level of interferenceexperienced by the subscriber station warrants such an allocation. Thebase station may also utilize a used subcarrier bitmap in order todetermine availability of subcarriers and times of availabilityoccurring during the coordinated DL zone. The used subcarrier bitmap maybe shared among a plurality of base stations.

Coordinating DL zones among multiple base stations permits subscriberstations to perform one or more interference mitigation techniques toimprove the quality of the received signals.

FIG. 5 is a simplified flow chart of an embodiment of a method 500 ofprocessing received signals in a coordinated downlink zone. The method500 can be implemented, for example, in a subscriber station, such asthe subscriber station illustrated in FIG. 3 operating in the wirelesscommunication system of FIG. 1.

The method 500 begins at block 510, where the subscriber stationexchanges capabilities with the serving base station of the coverage arein which the subscriber station resides. In particular, the subscriberstation may signal its ability to support coordinated DL zones. Thesubscriber station may signal its ability to support coordinated DLzones, for example, during a capabilities exchange with the basestation. The subscriber station can be configured to send atype/length/value message identifying coordinated DL zone capabilities.The message can include a type value identifying the message as acoordinated DL capabilities message. The length of the message can bepredetermined according to the type, and the values populating themessage may indicate the subscriber station capability to supportcoordinated DL zone with various permutation types.

After exchanging capabilities, the subscriber station proceeds to block520 and receives over the air (OTA) base station signaling messages. TheOTA base station signaling messages can be received from the servingbase station as well as one or more additional base stations that mayinadvertently operate as interference sources to the subscriber station.The one or more additional base stations will likely be neighbor basestations to the serving base station.

The OTA signaling messages can include, for example, messages thatidentify a DL permutation base and PRBS_ID for the transmitting basestation. The OTA signaling messages can also include a message thatindicates the use or lack of use of dedicated pilots, and the state of ause all subchannels field. Furthermore, the OTA signaling messages caninclude one or more downlink management messages that each identifiesthe presence and parameters associated with a coordinated DL zone. Theparameters may include, for example, the boundaries of the coordinatedDL zone and the permutation type utilized within the coordinated DLzone.

The subscriber station proceeds to block 530 and receives a downlinkresource allocation. The subscriber station can determine that it hasreceived a downlink resource allocation in any one of a variety ofmethods generally used in wireless communications. In one embodiment,the subscriber station can receive and decode a downlink MAP messagethat identifies the downlink resource allocations in a particular frameof data or series of frames of data.

The subscriber station proceeds to decision block 532 and determines ifthe downlink resource allocation falls within a coordinated DL zone. Thesubscriber station can, for example, compare the downlink resourceallocation to the boundaries of the coordinated DL zones that arereported or that can be determined from the downlink management controlmessages.

If the downlink resource allocation is not within a coordinated DL zone,the subscriber station proceeds to block 580 and does not implement theinterference mitigation techniques of the method 500. If the subscriberstation determines that the downlink resource allocation falls within acoordinated DL zone, the subscriber station proceeds to decision block540.

At block 540, the subscriber station determines the received power ofeach identifiable signal source, and ranks the signal sources accordingto received power. The subscriber station then determines apredetermined number, N, of the strongest signal sources for furtherprocessing. Ideally, the received signal from the serving base stationis included in the strongest signal sources. The number, N, of signalsand the number of interferers that are processed is only limited by thesubscriber station implementation. In a particular subscriber stationembodiment, the number of signals processed for interference mitigationmay be limited by a decoder structure utilized for processing signalswhen not in an interference limited link. For example, a MaximumLikelihood (ML) decoder configured to process two 64-QAM streams can bereconfigured to perform interference mitigation on the desired signaland signals from up to three interferers. In such an implementation, thehardware cost for implementing interference mitigation within thesubscriber station is minimized.

The subscriber station proceeds to block 540 and determines a channelestimate for each of the strongest signals. Thus, in the exampleprovided above, the subscriber station estimates the channel for thedesired signal and the three strongest interference signals. Thesubscriber station can utilize the parameters transmitted in the basestation OTA messages to perform channel estimation. For example, foreach base station signal for which the subscriber station generates achannel estimate, the subscriber station can utilize the DL_PermBasevalue and PRBS_ID value to estimate the channel based on the preamblebroadcast by the base station. The subscriber station can refine thechannel estimate derived from the base station preamble based on thepilot signals broadcast by each base station. The subscriber station canperform, for example, an iterative Minimum Mean Squared Error (MMSE)channel estimate for each monitored signal based on the base stationpreambles and the broadcast pilots.

The broadcast pilots, i.e. non-dedicated pilots, broadcast by each ofthe base stations supporting the coordinated DL zone purposely collidein time and frequency when the wireless communication system isconfigured for reuse-1. The subscriber station is able to determine achannel estimate for each of the desired signal and the predeterminednumber of strongest interference signals based on the coincident pilotsignals. Of course, the pilot signals need not be received coincident intime and frequency for the subscriber station to determine a channelestimate.

After determining the channel estimate for each of the desired signaland predetermined number of strongest interference signals, thesubscriber station proceeds to block 560. The subscriber station may nothave sufficient resources of be configured to estimate the channel forevery interferer. For example, the subscriber station described abovecan estimate the channel for three interferers, but there may beconditions in which there are more than three interferers. Thesubscriber station can treat the remaining weaker interferers as colorednoise. In one embodiment, the subscriber station mitigates theinterference contributed by the aggregate of the weaker interferers forwhich a channel is not estimated by exploiting autocorrelationproperties of the interferers. The subscriber station can, for example,modify the noise vector produced by the remaining weaker interferers toremove the colored aspect of the noise and effectively whiten the noisecontribution. The subscriber station can utilize a Minimum Mean SquaredError estimate of a compensation vector that whitens the colored noisecontributed by the remaining weaker interferers.

The subscriber station proceeds to block 570 and performs interferencemitigation or cancellation based on the channel estimates and thewhitened noise attributable to the interferers for which a channelestimate was not generated. Interference cancellation can be performeddistinct from or as part of the decoding process. If interferencecancellation is performed distinct from decoding, the subscriber stationproceeds to block 580 following interference cancellation.

At block 580, the subscriber station decodes the desired downlink signalusing the interference cancelled signal. Alternatively, the subscriberstation can utilize the channel estimates in conjunction with thewhitened noise component to decode the received downlink signal. Forexample, the subscriber station may utilize a maximum likelihood (ML)decoder that uses the channel estimates in conjunction with the whitenednoise component to decode the received downlink signal. By using thechannel estimates determined from the strongest coordinated basestations that contribute as interferers and the whitened noise from theremaining weaker base station interferers, the subscriber station isable to perform interference cancellation on a greater number ofinterferers than is possible based on interference cancellation based onweighting of different signals received via distinct antennas. That is,the interference cancellation technique described in the method 500 ofFIG. 5 has more degrees of freedom than are available in a weightedcombination of antenna signals.

FIG. 6 is a simplified flow chart of an embodiment of a method 600 ofdetermining a downlink resource allocation zone. The method 600 may beimplemented, for example, within a base station, such as a base stationof FIG. 1 or the base station of FIG. 2.

The method 600 begins at block 610 where a serving base station receivesone or more Channel Quality Indicator (CQI) values corresponding to oneor more CQI types from a subscriber station for which the base stationis the serving base station. The different CQI type can include, forexample, Signal to Interference Ratio (SIR), Interference to Noise Ratio(INR), Carrier to Interference Ratio (CIR), Carrier to Interference andNoise Ratio (CINR) and the like. The CQI type need not be a ratio, butcan be some other metric, such as, a Frame Error Rate (FER), a symbolerror rate (SER), a Packet Error Rate (PER), a Bit Error Rate (BER), andthe like.

The base station may receive the signals at the base station receiverand couple the CQI values to a coordinated DL zone manager forinterference analysis. The base station proceeds to block 620 andcompares the one or more CQI values corresponding to the one or more CQItypes to one or more thresholds. For example, each CQI type can have oneor more associated threshold against which the received CQI values arecompared.

The base station proceeds to decision block 630 and determines, based onthe CQI values and the threshold comparisons, whether the subscriberstation is interference limited. If not, the base station returns toblock 610 to process further CQI values. If the base station determines,at decision block 630, that the subscriber station is interferencelimited, the base station proceeds to decision block 542.

At decision block 642 the base station determines if the modulationcoding scheme for the subscriber station is variable and if suchvariation may improve the signal quality at the subscriber station. Forexample, the base station and subscriber station may support multiplemodulation coding schemes, and the channel quality may degradeperformance of a high order modulation coding scheme. The base stationmay determine that the subscriber station is better served using a morerobust modulation coding scheme and may initially attempt to improvesignal quality through modification of a modulation coding scheme beforeassigning the subscriber station to a coordinated DL zone.

If the subscriber station supports a more robust modulation codingscheme, the base station proceeds from decision block 642 to block 644,where the base station updates the selected or otherwise activemodulation coding scheme for the subscriber station to a more robustmodulation coding scheme. The base station returns from block 644 toblock 610 to process further CQI values.

If, at decision block 642, the base station determines that thesubscriber station does not support multiple modulation coding schemes,or that the presently active modulation coding scheme for the subscriberstation is supported by a coordinated DL zone, the base station proceedsto decision block 650.

At decision block 650, the base station determines if the subscriberstation is already assigned to a coordinated DL zone. If so, assigningthe subscriber station to a different coordinated DL zone may notimprove the signal quality to the subscriber station. The base stationproceeds back to block 610 to process further CQI values without makingany change.

If, at decision block 650, the base station determines that thesubscriber station is not already assigned to a coordinated DL zone, thebase station proceeds to block 660 and assigns the subscriber station toa coordinated DL zone. The base station may assign the subscriberstation to a coordinated DL zone by examining a used subchannel bit mapto determine available resources within a coordinated DL zone. The basestation may select from open subchannels and may allocate downlinkresources to the subscriber station within a selected set of opensubchannels within the coordinated DL zone. The base station returns toblock 610 after updating the resource allocation assignment.

The wireless communication system can be, for example, an OFDMA wirelesscommunication standard operating in accordance with an IEEE 802.16standard. The standard can be modified to support intercell interferencemitigation as described herein with little to no effect on legacydevices.

As described above, in order to provide adequate support to aid theimplementations of the interference mitigation schemes, the wirelesscommunication system can implement a coordinated downlink (DL) zone,which is a DL zone coordinated between the servicing base station (BS)and all its neighbor BSs, including, without limitation, the followingfeatures:

1. substantially the same zone boundary;

2. the same zone permutation type, e.g., PUSC (partial usage ofsubchannels), STC PUSC (space time coding partial usage of subchannels),AMC (adaptive modulation and coding), STC AMC (space time codingadaptive modulation and coding), and FUSC (full usage of subchannels).

In such a coordinated DL zone, the same zone boundary and the same zonepermutation type allow for the interference to be stable, and also allowthe interference properties of the neighbor BSs to be learned by usingpilot subcarriers, as the pilot subcarriers from all neighbor BSs aretransmitted at the same time and the same time and frequency locations.By gaining the knowledge of the interference properties from allneighbor BSs, the interference mitigation schemes can be effectivelyimplemented.

A frame can have zero, one, or multiple coordinated DL zones. In oneembodiment, the first PUSC zone can also be a coordinated DL zone.

A number of permutation types, such as PUSC, STC-PUSC, AMC, STC-AMC, andFUSC, are allowed to be used in a coordinated DL zone. Each permutationtype can have zero or one coordinated zone in a frame, except for thePUSC. For the PUSC, in addition to the first DL PUSC zone, there can beother coordinated DL PUSC zones.

To enable the interference mitigation schemes based on the coordinatedDL zone concept, supports are needed at air interface to signal thepresence of such coordinated DL zones and some PHY properties. The802.16 Rev2/D3 specification may be adapted or otherwise modified inorder to provide support for coordinated DL zone based signaling. Forexample, the current 802.16 Rev2/D3 specification may be amended by:

1. include a (type/length/value) TLV in a subscriber station basiccapability request (SBC-REQ) and the complementary subscriber stationbasic capability response (SBC-RSP) messages to signal the BS's supportfor the coordinated DL zones and subscriber station interferencecancellation capability in the coordinated DL Zones;

2. include a TLV in a downlink channel descriptor (DCD) message tosignal the presence and type of a Coordinated DL zone;

3. use a currently reserved bit in the DL zone switch informationelement (IE) to signal that the zone is a Coordinated DL zone;

4. include a TLV in a MOB_NBR-ADV message for each neighbor BS tospecify its DL_PermBase and PRBS_ID.

In summary, a few minor and backward compatible changes may beimplemented to the 802.16 Rev2/D3 standard to support the implementationof interference mitigation schemes using coordinated DL zones describedherein.

Methods and apparatus are described herein for intercell interferencemitigation in a wireless communication system.

As used herein, the term coupled or connected is used to mean anindirect coupling as well as a direct coupling or connection. Where twoor more blocks, modules, devices, or apparatus are coupled, there may beone or more intervening blocks between the two coupled blocks.

The steps of a method, process, or algorithm described in connectionwith the embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. The various steps or acts in a method or processmay be performed in the order shown, or may be performed in anotherorder. Additionally, one or more process or method steps may be omittedor one or more process or method steps may be added to the methods andprocesses. An additional step, block, or action may be added in thebeginning, end, or intervening existing elements of the methods andprocesses.

The above description of the disclosed embodiments is provided to enableany person of ordinary skill in the art to make or use the disclosedembodiments. Various modifications to these embodiments will be readilyapparent to those of ordinary skill in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the scope of the disclosure.

What is claimed is:
 1. A method of transmitting downlink (DL) data in awireless communication system comprising a plurality of base stations,the method comprising: receiving, at a base station, from a subscriberstation, channel quality information, wherein the channel qualityinformation comprises information indicative of received signal strengthof a transmission received from the base station at the subscriberstation; on a condition that the subscriber station requires reducedinterference levels, communicating, from the base station, informationto a neighboring base station to facilitate a coordinated DLtransmission zone that comprises downlink frequency and time resourcesthat are coordinated between the base station and the neighboring basestation, wherein the downlink frequency and time resources include afirst frequency and time resource that is available and useable by thebase station for a coordinated time period; updating, at the basestation, a used resource bitmap associated with the coordinated DLtransmission zone to indicate that the first frequency and time resourceis in use and sharing the used resource bitmap with the neighboring basestation; and transmitting, from the base station, data to the subscriberstation according to the first frequency and time resource during thecoordinated time period.
 2. The method of claim 1, wherein the downlinkfrequency and time resources include a portion of a downlink frequencyband.
 3. The method of claim 1, wherein the channel quality informationis a channel quality indicator (CQI).
 4. The method of claim 1, whereinthe base station supports a coverage area adjacent to the neighboringbase station.
 5. The method of claim 1, wherein the downlink frequencyand time resources include an allocation of a plurality of OrthogonalFrequency Multiple Access (OFDM) resources.
 6. The method of claim 5,wherein each OFDM resource is associated with a plurality ofsubcarriers.
 7. The method of claim 1, further comprising: allocating,by the base station, the first frequency and time resource to thesubscriber station for the coordinated time period.
 8. The method ofclaim 1, wherein the information includes a boundary, a permutationtype, and a pilot signal type for the coordinated DL transmission zoneand wherein the permutation type is at least one of partial usage ofsubchannels (PUSC), space time coding partial usage of subchannels (STCPUSC), adaptive modulation and coding (AMC), space time coding adaptivemodulation and Coding (STC AMC), or full usage of subchannels (FUSC). 9.The method of claim 8, wherein the pilot signal type is at least one ofdedicated pilot signaling and broadcast pilot signaling.
 10. The methodof claim 1, wherein the used resource bitmap is a subcarrier bitmap thattracks available and used subcarriers in the coordinated DL transmissionzone over time.
 11. A wireless communication system comprising: aplurality of base stations, each base station comprising: a transceiver;and a processor; wherein for a base station in the plurality of basestations: the transceiver is operable to receive, at the base station,from a subscriber station, channel quality information, wherein thechannel quality information comprises information indicative of receivedsignal strength of a transmission received from the base station at thesubscriber station; on a condition that the subscriber station requiresreduced interference levels, the processor is operable to communicate,from the base station, information to a neighboring base station tofacilitate a coordinated downlink (DL) transmission zone that comprisesdownlink frequency and time resources that are coordinated between thebase station and the neighboring base station, wherein the downlinkfrequency and time resources include a first frequency and time resourcethat is available and useable by the base station for a coordinated timeperiod; the processor is operable to update, at the base station, a usedresource bitmap associated with the coordinated DL transmission zone toindicate that the first frequency and time resource is in use andsharing the used resource bitmap with the neighboring base station; andthe transceiver is operable to transmit, from the base station, data tothe subscriber station according to the first frequency and timeresource during the coordinated time period.
 12. The wirelesscommunication system of claim 11, wherein the downlink frequency andtime resources include a portion of a downlink frequency band.
 13. Thewireless communication system of claim 11, wherein the channel qualityinformation is a channel quality indicator (CQI).
 14. The wirelesscommunication system of claim 11, wherein the base station supports acoverage area adjacent to the neighboring base station.
 15. The wirelesscommunication system of claim 11, wherein the downlink frequency andtime resources include an allocation of a plurality of OrthogonalFrequency Multiple Access (OFDM) resources.
 16. The wirelesscommunication system of claim 15, wherein each OFDM resource isassociated with a plurality of subcarriers.
 17. The wirelesscommunication system of claim 11, wherein: the processor is operable toallocate, by the base station, the first frequency and time resource tothe subscriber station for the coordinated time period.
 18. The wirelesscommunication system of claim 11, wherein the information includes aboundary, a permutation type, and a pilot signal type for thecoordinated DL transmission zone and wherein the permutation type is atleast one of partial usage of subchannels (PUSC), space time codingpartial usage of subchannels (STC PUSC), adaptive modulation and coding(AMC), space time coding adaptive modulation and Coding (STC AMC), orfull usage of subchannels (FUSC).
 19. The wireless communication systemof claim 18, wherein the pilot signal type is at least one of dedicatedpilot signaling and broadcast pilot signaling.
 20. The wirelesscommunication system of claim 11, wherein the used resource bitmap is asubcarrier bitmap that tracks available and used subcarriers in thecoordinated DL transmission zone over time.